Conventional well-known methods for providing corrosion resistance and coating adhesion with the steel material include zinc phosphate treatment and zirconium-based chemical conversion treatment.
The zinc phosphate treatment has been used for a long time as a chemical conversion treatment for a steel material. This zinc phosphate treatment is effective not only for the steel material but also for zinc-based materials and aluminum alloy materials. However, the solution used for the zinc phosphate treatment contains as its main component phosphorus which is a eutrophication element or nickel with the risk of carcinogenicity. In addition, this process is associated with the generation of a considerable amount of sludge. Accordingly, use of the zinc phosphate treatment is less favored in these days for environmental reasons.
In contrast, the zirconium-based chemical conversion treatment has recently received attention as a substitute for the zinc phosphate treatment since this method can be carried out with reduced environmental load. However, this method is originally a technique which as been used for an aluminum alloy material, and accordingly, it has been difficult to realize a sufficient coating weight on a steel material, and also, the coating adhesion and the corrosion resistance after the coating were not of the level realized in the zinc phosphate treatment. In view of such situation, various improvements have been proposed.
Exemplary improvements of the zirconium-based chemical conversion treatment for a steel material include the following Patent Literatures.
Patent Literature 1 discloses a chemical conversion agent comprising at least one member selected from zirconium, titanium, and hafnium, fluorine, and a water soluble resin wherein the water soluble resin comprises a constitutional unit represented by the following formula (1):
and/or the following formula (2):
in at least a part thereof.
Patent Literature 2 discloses a coating pretreatment comprising at least one member selected from the group consisting of zirconium, titanium, and hafnium, fluorine, and at least one member selected from the group consisting of an amino group-containing silane coupling agent, its hydrolysate, and its polymerization compound.
Such zirconium-based chemical conversion treatment can be conducted with reduced environmental load, and such treatment is also capable of improving the coating adhesion to the steel material as well as the corrosion resistance after the coating.    [Patent Literature 1] JP 2004-218074 A    [Patent Literature 2] JP 2004-218070 A